Woodchips or brush chips have been used in the paper manufacturing industry as well as in the manufacturing of particle board and oriented strand board for many years. For the purposes of this application, the terms “woodchips” and “biomass” should be considered to include woodchips, brush chips and other forms of biomass that are harvested from forests and wooded areas as well as crops that are grown with the specific intent for the crops to be harvested for biomass. Woodchips are commonly produced from forest products that are too small to be used for the production of lumber and are used as raw materials for the production of paper pulp in the paper industry as well as in the production of particle board and oriented strand board as mentioned above. Woodchips are commonly transported by truck. Trucks are used because of the necessity of being able to get the transportation vehicle into the woods or other areas where the woodchips are produced. The wooded locations where woodchips are produced are often remote and subject to regular relocation as the trees or brush that are converted to woodchips are harvested. Thus, trucks are the preferred transportation mode for woodchip transport.
Woodchips have a relatively low density as compared to other granular materials that are commonly transported by truck. Coal, gravel and sand are much denser and are commonly transported by dump trucks. Woodchips, because of their low density and the fact that they commonly have irregular sharp edge shapes, do not flow from the inside of a dump truck bed as readily as do coal, gravel or sand. Further, woodchips contain moisture and are often harvested in winter. The moisture and low winter temperatures can cause the woodchips to freeze to each other and to the inside of the truck and be difficult to remove from the truck. Biomass materials may also bridge the width of the truck and impede unloading.
To dump woodchips from a dump truck it is necessary to raise the truck bed to an extreme angle as compared to the angle sufficient to dump denser materials from a truck. When this is done in the context of longer truck beds such as semi-trailers, the extreme angle to which the truck bed is raised creates a serious risk that the raised truck bed will topple to one side or the other creating a safety hazard as well as the possibility of damage or destruction of the truck. Accordingly, truck dumpers are commonly used to empty trucks that are used to transport woodchips or other biomass materials.
A truck dumper includes a large tilt bed onto which the semi-trailer containing woodchips is positioned. Some truck dumpers operate by tilting only the semi-trailer uncoupled from its tractor, while other truck dumpers lift the entire tractor-trailer rig together for dumping.
The trailer is secured to the tilting bed platform, for example by clamps, and the tilting platform is raised, typically by hydraulic rams, to a position approaching vertical so that gravity can cause the contained woodchips or other biomass materials to exit the back of the truck bed.
Some truck dumpers permit the truck to be driven onto the tilt bed in a forward direction while others require that the truck be backed onto the tilt bed.
In the paper mill, particle board and oriented strand board industries, it is common to control the dust that arises from the dumping of woodchips or other biomass by spraying water to clear the dust from the air and cause the dust to sink to the ground. In these industries, the woodchips or other biomass are processed in such a way that added moisture is required. So, the addition of water for dust control does not cause a problem and may be a benefit to the production process.
More recently, it has become desirable to burn woodchips or other biomass for the production of electrical energy at power plants. Usually, the woodchips or biomass are added to coal that is burned to generate energy. Because biomass is a renewable resource, it is desirable to substitute it, at least partially, for fossil fuels such as coal. Biomass may also come from crop residues, dedicated energy production crops, forest and wood processing waste, livestock and poultry waste and urban waste as well as food processing residues. The addition of 15% biomass to coal burned for the production of electrical energy benefits the energy production industry because the carbon emissions created by this combination are considered to be no more than that of natural gas.
In the context of burning biomass for energy, it is undesirable to use water to control dust because it makes the biomass more difficult to burn, reduces BTU content of the biomass and because moisture in the biomass tends to increase the likelihood of spontaneous combustion because the biomass is stored in a relatively dry state. Spontaneous combustion is generally not a problem in the paper pulp, particle board or oriented strand board industries because the woodchips or biomass are commonly stored in a semi-liquid slurry where the moisture level is too high to permit spontaneous combustion.
In addition, wood dust entrained in the air can lead to a dust explosion. As discussed above wood and wood dust has a low density and floats in the air more readily than coal dust. When the concentration of wood dust is right a source of ignition can lead to devastating explosion.
Further, the cellulose particles in wood dust are an environmental pollutant and if breathed can contribute to health problem in individuals exposed.